The semitransparent atmospheric window between about 9.5 and 11.3 microns of wavelength is typically used to remotely measure the temperature of cloud margins and other targets and infrared emitters viewed through the atmosphere with noncontact infrared thermometers (see for example FIG. 1A illustrating exemplary detector sensitivity in that window). This window enables measurements with a minimum of error-inducing contributions of the intervening atmosphere (see, for example, of the low contributions attributable to water vapor and CO2 and other gases in FIGS. 2A and 2B, respectively). This has applications in surface-based upward looking and top-down and horizontal path passive microwave radiometry, weather observations and forecasting, weather and cloud physics research, military applications such as target discrimination and battlescene infrared visibility and other remote sensing applications wherein the true temperature of the remotely sensed target is to be accurately known.
In atmospheric radiometry applications a tropospheric profiling radiometer such as the RADIOMETRICS CORPORATION MP-3000A can be used to measure altitude profiles of temperature, water vapor, and cloud liquid water by interpreting the inwelling microwave emissions of the atmospheric constituency. Because this measurement method is a passive remote sensing method, the range or vertical resolution is not optimum. However, the temperature profile measurement has the advantage of being constrained by the Perfect Gas Law and the Hydrostatic Equation. Water vapor is not so constrained and can occur at most any altitude in highly structured distributions, constrained only by the temperature dependent saturation vapor density (see FIG. 3). Its spatial distribution is therefore much more difficult to accurately characterize. However, knowing the cloud margin temperature defines the saturation vapor density, and thus the vapor density at that altitude. Such knowledge of the temperature of the cloud base therefore constrains the vapor density at that location on the atmospheric temperature profile and improves the ability of a profiling radiometer to accurately profile atmospheric water vapor.
An infrared thermometer observing from 9.5 to 11.3 microns (see FIG. 1A) can be utilized to measure the infrared emission from, and therefore the temperature and saturation vapor pressure of, cloud margins. Whereas microwave wavelengths pass through clouds, at such infrared wavelengths the emission originates from a shallow thickness of the cloud margin. The atmosphere between the infrared thermometer and cloud margin contributes only a small amount to the received cloud or emitter signal between 9.5 and 11.3 microns (see NB in FIGS. 2A and 2B). This invention offers a method of correcting out this contribution, thereby giving a more accurate temperature datum of the emitter.
Infrared thermometers with this narrowed bandpass (narrowband infrared thermometers) are expensive and very limited in choice. Commercially available narrowband infrared thermometers are also typically less robust than wideband infrared thermometers. Wideband infrared thermometers operating in the range of about 8 to 14 microns (see the exemplary detector sensitivity chart of FIG. 1B) are inexpensive, about $500 as compared to $6000 for narrowband infrared thermometers currently utilized in this application. However, a significant fraction of the signal within the 8 to 14 micron region outside of the 9.5 to 11.3 micron waveband comes from atmospheric water vapor (see WB in FIG. 2A) and dry constituents such as CO2, ozone, methane, N2O, NO2, and other trace gases (see WB in FIG. 2B) in the atmospheric interval between the infrared thermometer and cloud base.
It would thus be useful to enhance accuracy of temperature measurements by correction of infrared thermometer readings (in either or both narrowband and wideband instruments) for these intervening atmospheric components. Further, in view of the costs with use of narrowband instruments, means and methods for use of wideband infrared thermometers in applications calling for accurate measurements normally not achievable by such instruments would be advantageous. Because provision of narrowband infrared thermometers presently represent a significant fraction of the cost of tropospheric profiling microwave radiometer and other profiling and measurement systems, the ability to utilize a lower cost wideband infrared thermometer would be especially attractive in such applications.